1. Field of the Invention
The present invention generally relates to an apparatus and a method for reducing vibration and acoustic noise when performing a high-speed positioning operation of a transducer such as a magnetic head, an optical head and a print head etc. More particularly, the present invention relates to an actuator and related method of operation that is controlled so as to reduce the harmonic content and amplitude in a control current for controlling its acceleration.
2. Description of the Related Art
In order to read data from or write data to a hard disk drive (HDD) system, a read/write head in the HDD must be accurately positioned over a desired track and sector of a magnetic recording disk in the HDD. This is generally done through the means of a seeking operation. As the speed of computers increases, however, it becomes more and more necessary to decrease the time allowed for positioning the read/write head, so that the access speed of data in the HDD is increased. As a result, there is a continued pressure to decrease the time required for a seeking operation, while maintaining the accuracy of the operation.
In performing seek operations, conventional disk drive devices generally use a multi-mode algorithm to meet wide dynamic control ranges. A short seek operation generally includes three control modes: a linear mode, a settle mode, and an on-track mode. A long seek operation generally includes at least five or six control modes including: an acceleration mode, a coast mode, and a deceleration mode, as well as the three linear mode, settle mode, and on-track mode from a short seeking operation. The coast mode may or may not be required, depending upon the length of the long seek operation.
A short seek operation is generally performed when the seek distance is extremely short, generally from three to five tracks, depending upon the specific HDD system implementation. Short seek operations and the three modes used in them are well known in the art, and so will not be discussed in detail here.
A long seeking operation uses the additional three modes (acceleration, coasting, and deceleration) to position the read/write head from any given location to the vicinity of the desired track, i.e., within about three to five tracks. The next three modes (linear, settle, and on-track) are then used for a finer adjustment of the position of the read/write head.
During the acceleration mode, an actuator that holds the read/write head is accelerated up towards a maximum velocity as it moves towards the desired track. Once the read/write head reaches the maximum velocity, the system enters the coast mode and the read/write head coasts for a time at the maximum velocity. Generally, the system carries out a closed-loop velocity control to keep the read/write head moving at the maximum velocity. When the read/write head reaches the proper location, the system enters the deceleration mode to decelerate the read/write head so that it comes to rest in the vicinity, i.e., within three to five tracks, of the target track.
Of course, for a short enough long seeking operation, the coast mode may not be needed, since the read/write head may not reach its maximum velocity before it's time to decelerate. In this case, the seeking operation will enter an acceleration mode to move to the read/write head to a lesser velocity and will then enter a deceleration mode to bring the read/write head to rest in the vicinity of the target track.
The first three modes in a long seeking operation (acceleration, coast, and deceleration) are primarily responsible for any vibration or acoustic noise that the system experiences. These three modes are implemented through a velocity-tracking structure in conventional disk controllers. In these velocity tracking structures, an estimator is used to construct a velocity feedback signal since there is no velocity feedback sensor in normal systems. In this estimator, a velocity trajectory generator is used to generate a velocity trajectory as a function of the distance remaining for the read/write head to travel. The fastest trajectory is then determined by driving the HDD motor's inertia and torque constant. If this trajectory is used, it constructs a time optimal controller, which can also be called a Bang--Bang controller.
In other words, the actuator uses a maximum current to accelerate the system and uses a maximum current to decelerate the system. This controller is not practical, however, because it is too sensitive to position error and to noise. Therefore, many modifications have been used to obtain more stable positioning and better noise control.
One conventional hard disk drive seek operation is performed as follows. First, a target track is input into a control system and the control system determines whether a long or short seek is required to move from the present track to the target track. As noted above, a short seek operation is used if the seek distance, i.e., the distance from the current track to the target track, is at or below a given threshold, e.g., three to five tracks. Otherwise, a long seek operation is used.
If a long seek is required, a voice coil motor (VCM) in the HDD is supplied with a current based on a square-like wave acceleration seek trajectory profile to accelerate the read/write head up to a maximum velocity. The current to the VCM is then stopped when the maximum velocity is reached and the system enters a coast mode in which the read/write head coasts at the maximum velocity. Then, when the read/write head reaches the proper track position, it begins to decelerate by receiving a square-like wave deceleration seek trajectory profile.
If the long seek is of a short enough distance that the maximum velocity is not reached, the coast mode is omitted. In this case, the VCM in the HDD is supplied with a square-like wave acceleration seek trajectory profile to accelerate the read/write head up towards the maximum velocity. However, before it reaches the maximum velocity, the VCM is then stopped and begins to decelerate based on the square-like wave deceleration seek trajectory profile.
Once the read/write head is within three to five tracks of its target track, whether by acceleration, coasting, and deceleration in a long seek, or at the start if the seek operation is a short seek operation, the system will enter the linear mode directly. After it completes the linear mode, the system enters a settle mode and an on-track mode.
FIGS. 1A and 1B are examples of conventional seek trajectory profiles. In particular, FIG. 1A is a graph showing a conventional square-wave seek trajectory profile with a coast mode; and FIG. 1B is a graph showing a conventional square-wave seek trajectory profile without a coast mode.
The acceleration in a seeking operation is achieved by running a current to the actuator in the HDD. As shown in FIGS. 1A and 1B, in this conventional operation, the current is provided as a square-like wave. In FIG. 1A, the acceleration rises until the maximum velocity is reached, levels off to zero during the coast mode, and drops to a maximum deceleration before slowly rising again until the actuator arm comes to a stop.
However, the VCM is a very rigid structure, having a natural frequency above 3.5 KHz. As a result, a high frequency of operation above 3.5 KHz will excite this natural frequency and cause resonance. This means that in a long seek using a square-like wave, to control the VCM, the assembly may resonate. And if the resonance cannot be quickly decayed, it will have an undesirable effect on track accuracy.
In addition, the VCM assembly itself moves very fast, having a bandwidth above 550 Hz. As a result, the backward-forward motion of the assembly itself causes vibrations and acoustic noise.
Because of these problems, numerous efforts have been made in conventional disk drive devices to reduce vibration and acoustic noise. These efforts at reducing vibration and acoustic noise can be categorized generally into three approaches: the use of mechanical designs, the use of damping material, and the use of a special seeking trajectory profile.
When employing mechanical designs, the structure of a read/write head and the associated HDD equipment are modified to reduce the noise generated by the motion of the read/write head. In a conventional HDD system, a read/write head is fixed to a transducer suspension arm and actuator. This assembly structure can be designed so as to be so rigid that its resonance frequencies are extremely high.
Normally, those frequencies are chosen to be sufficiently high to be far away from the servo bandwidth. However, with the increasing density of data storage systems, correspondingly higher servo bandwidths are required, which in turn demands higher resonance frequencies. Therefore, using currently-available materials and structures, both mechanical design and related servo design are being pushed to the maximum of their capabilities, with a corresponding increase to their cost.
As these servo bandwidths continue to rise, it becomes very difficult to predict whether the mechanical resonance frequencies can be pushed far enough away from the servo bandwidth to be effective. As result, there is a higher possibility that the mechanical structure, with its limited resonance frequencies, will be excited by rich harmonic content in a conventional, "square wave-like" control currents.
One other approach for reducing vibration and acoustic noise is the use of damping materials in a hard disk drive. Damping materials can be used to absorb some resonant energy, but they are significantly limited by the structure of the data storage systems. Common damping materials are mostly made of a rubber-like material, which can be very difficult to assemble into hard disk drive housings. In addition, hard disk drive housings must be very clean inside, and housings made with damping materials cannot meet such stringent cleanliness requirements. As a result, although damping materials can reduce vibration and acoustic noise, they generally cause more problems than they solve.
Finally, efforts have been made to reduce vibration and acoustic noise using special seeking trajectory profiles. The use of a special seeking trajectory profile has proven to be the most worthwhile approach to eliminate vibration and acoustic noise in a hard disk drive system. Its basic principle is to look for an acceleration trajectory profile with a narrow harmonic content and a lower amplitude that prevents any potential vibration from the transducer that has the same seeking time as a "square wave-like" control input.
In U.S. Pat. No. 5,151,639, optimal control theory is used to obtain a special acceleration trajectory that mathematically minimizes the derivative of acceleration (or the current that drives the acceleration) in a seek head, i.e. di/dt. This trajectory is similar to a sinusoidal wave, but the algorithm is extremely complicated for a microcomputer to implement it due to the presence of six order polynomial equations involved in its calculation.
U.S. Pat. No. 5,465,034 offers another method for further reducing the harmonic content in a control current. In the disclosed system, a symmetrical sinusoidal-like wave is developed at the expense of seek time. This sinusoidal-like wave is specially designed to realize both symmetrical and single mode operation. FIG. 2 is a block diagram showing the processing of the projected position, velocity, and acceleration of a conventional system, and FIGS. 3A and 3B show examples of sinusoidal-like waves from this design.
As shown in FIG. 2, an analog actual position signal produced from an actuator 5 indicates the actual location of an HDD actuator read/write head with respect to the tracks on a hard disk. This signal passes through a sampler 10 and an analog-to-digital converter 15 to produce a digital actual position signal indicative of the measured actual location of the read/write head. This actual position signal is provided to a velocity trajectory generator 20 and a state estimator 25.
The velocity trajectory generator 20 generates a velocity trajectory profile in response to the actual position signal, and provides this profile to a positive input of a first adder 30. The state estimator 25 generates an estimated velocity based on the actual position signal and a control signal, and provides this estimated velocity to a negative input of the first adder 30. The output of the first adder 30 is provided to a first state controller 35, which in turn provides its output to a first positive input of a second adder 40.
The feed forward generator 45 uses the measured position signal to generate a feed forward position vector. This signal is sent to a second state controller 50, which provides its output to a second positive input of the second adder 40.
The second adder 40 generates the control signal, which is sent to both the actuator 5 and the state estimator 25. On its way to the actuator 5, the control signal passes through a digital-to-analog converter, such as a zero order hold (ZOH) 55. The control signal acts to control the operation of the actuator 5.
FIGS. 3A and 3B show examples of the sinusoidal-like waves that results from the prior art. As seen in FIGS. 3A and 3B, there is a very slow acceleration at the beginning of the waveform. For symmetry, a very slow deceleration can also be seen at end of the waveform. As a result, this causes long seeking time, which is very difficult to be adapted into most high performance data storage systems. In addition, this method uses a large look-up table and involves the use of a sorting algorithm, which can further burden the real-time operation of a microcomputer or digital signal processor.
In reality, there are two physical restrictions in maximum head motion velocity and the maximum current to the actuator. The maximum current is limited by the coil of the VCM and the integrated circuit that controls the VCM. The maximum velocity is limited by the head's ability to reliably read from the disk. The velocity must be kept at a level where data can still be read by the head. These restrictions directly affect the acceleration trajectory, whether in conventional "square wave-like" systems, or in the "sinusoidal wave-like" systems described above.
However, in the conventional methods described above, there is no information released relating with how to deal with those two limitations. As a result, these two methods have been limited in practice.